Gas chromatography is used to analyze and detect the presence of many different substances in a gaseous sample. Gas chromatography uses various types of detectors, depending on the specific element or compound sought to be detected. Different detectors are used to achieve selective and/or highly sensitive detection of specific elements or compounds, in particular chromatographic analyses.
Typically, a flame photometric detector is used to detect the presence of sulfur or phosphorous in a particular sample, or analyte. A flame photometric detector uses what is referred to as a chemiluminescent reaction where compounds containing sulfur or phosphorous encounter a hydrogen-rich flame. Chemiluminescence uses quantitative measurements of the optical emission from excited chemical species to determine analyte concentration. Chemiluminescence is typically emission from energized molecule species. When burned, or combusted, in such a flame, sulfur is transformed into an emitting species referred to as “S2” and phosphorous is transformed into an emitting species referred to as “HPO.” The emission wavelength range for excited S2 includes, among others, the region from 320-405 nanometers (nm) and the wavelength range for excited HPO includes, among others, the range from 510-530 nm. The molecular emissions impinge on a photomultiplier tube, which converts photons to an electrical signal to quantify the concentration of a particular excited species.
To selectively detect either the excited S2 emission or the excited HPO emission, a narrow band-pass optical (interference) filter has typically been used between the flame and the photomultiplier tube to isolate the appropriate emission band. Unfortunately, a narrow band-pass optical filter limits the signal-to-noise ratio, and therefore the signal strength of the signal delivered to the photomultiplier, resulting in an inability to detect minute quantities of an analyte. For example, a narrow band-pass optical filter used in a conventional flame photometric detector to detect sulfur transmits a photon emission band ranging in wavelength from 385-400 nm and has 65% transmissivity. A disadvantage of such an optical filter is that it only passes one of many characteristic emission bands for sulfur, thereby limiting the signal supplied to the photomultiplier tube. Further, to detect the presence of different analytes, different filters must be interchanged.
Therefore, it would be desirable to detect the presence of multiple elements without having to change interference filters in a flame photometric detector.